Helen Gurley Brown, the author and journalist credited with introducing frank discussions of sex into magazines for women during her 32-year career as the editor of Cosmopolitan, has died at the age of 90.

Brown was already at the forefront of influencing a change in sexual mores when she wrote Sex and the Single Girl, published in 1962, about her single life, encouraging women to have sex freely, regardless of their marital status.

But it was at the helm of Cosmopolitan, which she transformed from being an often straitlaced publication aimed at suburban housewives into one that built a global readership based on Brown and her colleagues' idealised image of the sexually liberated, career-focused "Cosmo girl".

New York City mayor Michael Bloomberg said the city had "lost a pioneer who reshaped not only the entire media industry but the culture of the US". He added: "She was a role model for the millions of women whose private thoughts, wonders and dreams she addressed so brilliantly in print. She was a quintessential New Yorker: never afraid to speak her mind and always full of advice. She pushed boundaries and often broke them, clearing the way for younger women to follow in her path."

Born into a family of modest means in rural Arkansas, Brown became a writer of advertising copy on the US West Coast after working at a string of secretarial jobs.

She was 37 when she married twice-divorced David Brown, a former Cosmopolitan managing editor turned movie producer who encouraged her to write a book. Sex and the Single Girl, her collection of advice, opinion and anecdote on why being single shouldn't mean being sexless, made her a celebrity and became one of the top sellers of 1962 in the US. Five more books followed, including Having It All in 1982 and in 1993, at age 71, The Late Show, which was subtitled A Semiwild but Practical Survival Plan for Women Over 50.

Brown was hired three years later by Hearst Magazines to turn around Cosmopolitan, declaring that her aim was to tell a reader "how to get everything out of life – the money, recognition, success, men, prestige, authority, dignity, whatever she is looking at through the glass her nose is pressed against".

Titillating cover lines became synonymous with the title, where sales grew every year until peaking at just over three million in 1983, before slowly levelling off to 2.5 million, where it was when Brown left in 1997.

Brown's focus on sex and approval of cosmetic surgery made her a controversial figure among many who suggested she was saying women should work the system rather than overthrow it.